In a rolled metal material such as a rolled copper alloy, a work affected layer (a layer in which metal crystal grains are finely formed compared with crystal grains inside the metal) is formed at a surface due to strong stress applied to the surface by a reduction roll during working. Then, most of deformation caused by internal stress is accumulated in the work affected layer as residual stress.
When the work affected layers in which residual stress is accumulated are equally formed at both surfaces of a metal material, the metal material can retain flatness as a plate material. However, if working by which the work affected layer at one side is partially or entirely removed is performed, the residual stresses at the both surfaces become unbalanced and the metal material becomes warped, that becomes an obstacle in manufacturing a lead frame.